(1) Field of the Invention
The present invention relates to a static-type semiconductor memory device in which the falling characteristic of the word selection line (word line) potential when the word line is transferred from the word selecting state to the word non-selecting state is improved.
(2) Description of the Prior Art
To keep the electric power consumption from rising along with the increasingly larger scale of semiconductor memory devices using bipolar random access memories (RAM), proposals have been made to reduce the holding current of the memory cells. A reduced holding current, however, results in slower falling time of the word line potential from the selecting state to the non-selecting state, therefore, obstructing high-speed memory operation.
To prevent the slowdown of the falling time of the word line potential, use has hitherto been made of the devices shown in FIGS. 1 or 2, indicated in the "Brief Description of the Drawings".
The device shown in FIG. 1 is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-41968(1978). In FIG. 1, MC is a flip-flop type memory cell, WL is a word line, HL is a hold line, forming a pair with the word line WL, B and B are a pair of bit lines, WD is a word driver, and I.sub.H is a holding current source. The memory cells MC are arranged in a matrix and form a memory-cell array. A memory cell MC is selected by turning on one of the word drivers WD and one of the bit drivers (not shown). In FIG. 1, the memory array is partially abbreviated using broken lines to simplify the drawing.
A word line discharge circuit 1 which improves the delay characteristic of the falling time of the potential when the word line WL turns from the selecting state to the non-selecting state is arranged for each word line WL. When one of the word lines WL is selected and is of a high potential, both transistors 11 and 12 belonging to the word line discharge circuit 1 and connected to the selected word line WL turn on and are in a conductive state. Then, the current I.sub.DIS flows through the transistor 12, making the total electric current through the memory cells connected to the concerned word line WL equal to the holding current and the current I.sub.DIS. The addition of the discharge current I.sub.DIS to the holding current when the word line is turned from the selecting state, to the non-selecting state eliminates the delay of the falling time from the high level to the low level at the word line. The discharge circuit used in this device has the merit of allowing amount of the discharge current I.sub.DIS to be determined independent of the operation of other word line circuits. The discharge circuit, however, has the problem of a greater possibility of an operation error due to fluctuations of the power source voltage V.sub.cc.
The device shown in FIG. 2 was disclosed in Japanese Unexamined Patent Publication No. 56-37884 (1981). The circuit of the device is the same as the circuit of the device in FIG. 1, except for the discharge circuit 2. In the discharge circuit 2 of this device, when the word line WL is driven to a high level, a transistor 21 enters the conductive state, thus the base of a transistor 22 rises to a high level. The transistor 22 then enters the conductive state and the discharge current flows from the constant current source. The addition of the discharge current to the holding current in the selecting state of the word line, improves the delay characteristics of the falling time of the word line potential at the moment the change from the selecting state occurs, to non-selecting state as in the device of FIG. 1. In this circuit, there is no problem of an operation error due to fluctuations of the power source voltage. Since the emitter circuits of the transistors 22, one of which is located at each hold line, are connected together to one constant current source, the operating conditions differ along with the characteristics of the individual transistors 22, particularly the value of h.sub.FE. Therefore there is a problem in that the transistor 22 for the selected word line does not always enter the conductive state alone.